1. Field of the Invention
The present invention concerns a circuit arrangement and method for amplification of an electrical input signal and a magnetic resonance system with such a circuit arrangement.
2. Description of the Prior Art
Amplification circuits are known that have a signal splitter that splits the input signal into a first partial signal and a second partial signal. A first signal path has a first amplification stage for amplification of the first partial signal and a second signal path has a second amplification stage for amplification of the second signal. Each of the two amplification stages is supplied with current from a power supply device. A signal combination element downstream from the amplification stages recombines the two partial signals into one output signal. The invention furthermore concerns a corresponding method for amplification of an electrical input signal.
Such circuit arrangements are in particular used in magnetic resonance systems for amplification of the radio-frequency pulses necessary for imaging. The amplification ensues with what a method known as a coherent power combination method, in which the signals to be amplified are superimposed with the amplification signals in phase, with not only the amplification of the signal being achieved on average but also the peak voltage is actually amplified. Very large amplifications of radio-frequency pulses are thereby possible, as they are required within magnetic resonance systems.
The amplification stages typically used in such circuit arrangements use power transistors designed on a semiconductor basis. One problem is that such amplification stages should not be operated above a maximum barrier layer temperature (junction temperature) of the transistors since this can lead to a premature failure of the amplification stage. The current junction temperature of the amplification stage depends on the current or on the current output power. In the amplification stages typically used in amplification devices in magnetic resonance systems, for example, the junction temperature cannot be more than 120° C. If this temperature is observed, the lifespan of the amplification stages is approximately 10 to 15 years.
If the junction temperature of 120° C. is observed, the output power is maximally limited to approximately 10 to 15 kW, depending on the type of amplification stage. In order to achieve a greater output power, as described above a number of amplification stages are combined in parallel in a circuit arrangement. This means that the signal to be amplified is initially divided (preferably symmetrically) in a signal splitter such that each partial signal exhibits only half of the output power. These partial signals are then maximally amplified by respective amplification stages and the amplified signals are subsequently added together again so that ultimately two times the power of the maximum output power achievable by the individual amplification stages can be achieved.
Due to tolerances in the transistors, it disadvantageously occurs that the amplification stages require different currents. This means that the amplification stages used within the circuit arrangement heat differently and thus possibly must be deactivated earlier when they reach the maximum allowed junction temperature. This mechanism leads on the one hand to a situation that the power loss in the circuit arrangement increases and its efficiency consequently decreases. It can additionally lead to an asymmetrical output power of the individual amplification stages, which leads to additional power losses. When this problem has previously been encountered in the field, the circuit has been provided with a cooling arrangement or the number of the power transistors is increased. Both methods significantly increase the energy expenditure and the costs for the amplification.